As disclosed in U.S. Pat. No. 4,720,532 (Seyferth et al.-I), it is known that silicon nitride precursor polymers have many uses which make it desirable for them to be soluble in at least some organic solvents, as well as to be capable of providing a high ceramic yield when pyrolyzed.
Various techniques have been used to prepare silicon nitride precursor polymers, including the reaction of ammonia with certain silanes which may be defined as follows:
______________________________________ dihalosilanes SiH.sub.2 X.sub.2 dihalodisilanes Si.sub.2 H.sub.4 X.sub.2 trihalosilanes SiHX.sub.3 alkyltrihalosilanes RSiX.sub.3 alkyldihalosilanes RSiHX.sub.2 tetrahalosilanes SiX.sub.4 ______________________________________
However, these known techniques lead to the formation of precursor polymers which have certain disadvantages. For example:
(A) Seyferth et al.-I, who prepare their precursors by subjecting a mixture of 20-1 molar proportions of an alkyldihalosilane and 1-20 molar proportions of a trihalosilane or alkyltrihalosilane to coammonolysis in an organic solvent, obtain polysilazanes which provide ceramics containing a substantial amount of carbon, as do Arai et al. (U.S. Pat. No. 4,818,611), whose precursors are prepared by the ammonolysis of one or more dihalosilanes or dihalodisilanes in an organic solvent;
(B) Seyferth et al.-II (U.S. Pat. 4,397,828), who prepare their polysilazanes by reacting a dihalosilane with ammonia in an organic solvent, form precursors which are unstable at room temperature and provide ceramics containing excess silicon; and
(C) Crosbie et al. (European patent application 0 242 188), who react a tetrahalosilane vapor with liquid ammonia, form products which are limited in their use because of their intractability.
What is still needed are silicon nitride precursors which: (1) have a chemical composition such as to make them capable of providing a high yield of silicon nitride ceramic that is substantially free of carbon, excess silicon, and/or oxygen and consequently could be expected to have improved electrical properties and, (2) also have the advantage of being tractable polymers that can be easily processed when used to form molding compositions, coating compositions, fibers, infiltrants, etc.
Such polymers should be soluble in common organic solvents and would most desirably be stable at room temperature so that they could be isolated from their reaction mixtures, stored at room temperature, and then redissolved in the desired solvents. Moreover, although higher molecular weight oils having such characteristics might be desired for such applications as injection molding, it would also be desirable for other applications to provide solids having such properties.